Precise control of electromagnetic energy on a deeply subwavelength scale in the near field regime is a fundamentally challenging problem. Generally, subwavelength scales are defined as length scales smaller than the free space wavelength of an electromagnetic wave oscillating at a given frequency. More specifically, in the RF frequencies used in RFID (nominally 1 GHz, although it covers a range at least as wide as 100 kHz to 10 GHz), the wavelength is in the range of 30 centimeters, so the subwavelength scale includes distances smaller than this. The ability to control and focus energy on a subwavelength scale is necessary for innovation in technologies, for example, such as near field imaging, near field communication (NFC), radio frequency identification (RFID), and wireless power transfer. These examples rely on manipulation of electromagnetic energy on a subwavelength scale, and would benefit greatly from techniques that allow manipulation of this energy on scales that are smaller than a wavelength.
Manipulation of electromagnetic (EM) energy in the subwavelength regime is a topic of broad interest to physicists and engineers alike. In the far field regime, a technique used to focus energy (with resolution even below the classical diffraction limit) at a desired point in space and time is known as the electromagnetic time reversal (TR). Electromagnetic time reversal (TR) refers to the invariance of Maxwell's Equations even when time, t, is replaced by its negative, −t, which still produces a consistent set of Maxwell's equations. As a result of reciprocity, the evolution of the fields between a source and receiver can be “rewound” in time by using the aforementioned “−t” substitution. Thus, signals observed by a receiver or receiver array can be recorded, reversed in time, and then rebroadcast into the channel from which they came where they then retrace their steps back to the source and constructively interfere to focus in space and time.
It is against this background that the techniques described herein have been developed.